The present invention discloses a new .alpha.-keto amide derivative or a salt thereof represented by the formula (1): ##STR1## wherein X represents (1) a peptide residue or amino acid residue in which the functional group may be protected, (2) a hydrogen atom or (3) an amino-protective group, Y represents a peptide residue or amino acid residue in which the functional group may be protected, E represents a substituent on the alkylene (C.sub.n H.sub.2n-1) and is a halogen, lower alkoxy or hydrogen, A represents a carbonyl group or mono- or di-substituted methylene (the substituent being a hydroxy, lower alkoxy, phenylimino or benzylimino), and n represents an integer of 2 to 6.The compound of the present invention is effective in inhibiting prolyl endopeptidase activity and useful as an active ingredient of an enzyme inhibitor.